Libnet  A C library for portable packet creation 

domains of sockets and protocols.

A C library for portable packet creation 

Included are also simple example test programs

Libnet is a collection of routines to help with the construction
and handling of network packets. It provides a portable framework
for low-level network packet writing and handling.

Libnet features portable packet creation interfaces at the IP layer
and link layer, as well as a host of supplementary and complementary
functionality.

WWW: http://www.infonexus.com/~daemon9/Projects/Libnet/